My house had finlock guttering, I had a similar problem but to the back of my house. I rang several companies who gave me contrasting prices for the work but I found the best quotes for front and back were from a company called finlock solutions. I paid 2900.
The flashing sealing the joint between your chimney stack and roof may have cracked or become dislodged, allowing water through the joint. You'll need to refit the loose flashing, or if it's damaged, replace it with the same type of flashing or a self-adhesive flashing strip.
A damp patch or mould has formed at the top of an upstairs wall Blocked or broken gutters will send water gushing down your wall, and possibly saturate it. You'll need to clear your gutters or repair the damaged ones.